Non-small cell lung cancer (NSCLC) is a highly lethal disease. Despite dose escalation with conformal radiotherapy (RT), there is still a high intrathoracic failure rate. A novel approach is needed to improve RT effectiveness in these tumors. Galectin-1 (Gal-1) is a secreted carbohydrate binding lectin that is well known for its rol in modulating T cell homeostasis. More recently, it has been shown to play a major role in cancer progression. It is expressed in many cancers, including NSCLC, and its expression has been correlated with aggressive tumor behaviors. Gal-1 has been implicated in several pathologic processes including tumor proliferation, aggregation, adhesion, migration, angiogenesis and enhancing T-cell apoptosis, which can, in turn, can confer tumor immunity. Using proteomic analysis, we found that Gal-1 secretion was enhanced by hypoxia and more recently by RT. Applying a combination of down-regulating Gal-1 in a non-NSCLC cell line and knocking-out the gene in host mice, we show that tumor-derived Gal-1 is more important than host- derived Gal-1 in promoting tumor growth and spontaneous metastasis (Appendix 3). Further mechanistic studies suggested that Gal-1 mediated its tumor promoting function by protecting hypoxic tumor cells from apoptosis while enhancing intratumoral T-cell death. We also found that Gal-1 is required for XPB1 splicing, a regulator of the unfolded protein (UPR) pathway. The UPR is a well-known pro-survival pathway reported by several investigators to protect cancer cells from hypoxia-induced cell death. Based on these data, it is logical to ask the following questions: (1) does the Ire1/XBP1 pathway mediate Gal-1 induced tumor cell apoptosis and (2) does targeting Gal-1 with RT and chemotherapy increase tumor control in hypoxic NSCLC? A major goal of this study is to understand the pro-survival function the Gal-1 in hypoxic tumor cells and to determine whether targeting this protein will increase RT efficacy in hypoxic NSCLC. A common clinical observation is that RT can often cause profound lymphopenia in cancer patients & the exact mechanism for such an effect is unknown. We hypothesize that RT induces Gal-1 secretion, which in turn causes global T-cell apoptosis, resulting in lymphopenia. A secondary goal of this proposal is to study the role of Gal-1 in RT-mediated lymphopenia and to determine whether blocking Gal-1 will ameliorate this affect. We will address the above stated goals using 3 specific aims. In Aim 1, we will evaluate the role of Ire1- XBP1 pathway in Gal-1 mediated tumor cell apoptosis under hypoxia and determine whether XBP1 activation is sufficient and required for this process. In Aim 2, we will determine whether repression of Gal-1 expression or inhibiting its function (blocking antibody or inhibitory disaccharide) will enhance RT and cisplatin chemotherapy effect in NSCLC cell lines and orthotopic mouse tumors. In addition, we will determine whether this effect is dependent on tumor oxygenation and on having an intact T-cell function in mice using T-cell depletion experiments. Finally, in Aim 3, we will establish whether Gal-1 tumor or host expression is required and necessary for RT-induced lymphopenia using a combination of genetically matched Gal-1 deficient tumors and Gal-1 deficient host. We will also correlate circulating Gal-1 level with peripheral T-lymphocyte levels in patients undergoing RT for solid cancers. These studies will help to establish that Gal-1 is a novel target that can be used in conjunction with RT to improve the treatment outcome in NSCLC.